Pressure-sensitive adhesives made from either hydroxyl-terminated polymers or random-hydroxylated polymers are known in the art. U.S. Pat. No. 3,515,773 discloses preparation of pressure-sensitive adhesives from (A) a hydroxyl-terminated diene polymer, and (B) an isocyanate-terminated polyether prepolymer. U.S. Pat. No. 3,532,652 teaches preparation of pressure-sensitive adhesives from (A) an acrylate polymer containing a randomly polymerized hydroxylated monomer, and (B) an isocyanate-terminated polyester or polyether prepolymer. Such adhesives typically are deficient in one or two of the properties of rolling ball tack, 180.degree. peel adhesion and shear adhesion. These deficiencies are remedied by use of a polymer containing both random and terminal hydroxyl functionality as component (A) of the pressure-sensitive adhesive.
Existing processes for production of hydroxyl-terminated liquid polymers are typically two-stage processes, e.g., the processes of U.S. Pat. Nos. 3,712,916 and 3,699,153. An improved one-step process involves use of a hydroxyl-containing disulfide to produce hydroxyl-terminated liquid polymers having sulfide linkages near the terminal portions of the polymer molecule. Some random hydroxyl functionality is also introduced by including small amounts of hydroxyl-containing monomers such as 2-hydroxyethyl acrylate in the monomer mixture.
Further improvement in the manufacture of the random and terminal hydroxylated liquid polymer is attained using a mixture of a small amount of at least one hydroxyl-containing trisulfide together with the hydroxyl-containing disulfide. Such a disulfide-trisulfide mixture permits viscosity control during polymerization with substantially less disulfide required than if the disulfide alone were used, and the resulting hydroxylated polymers have less odor and discolor less upon heating than if the disulfide alone were used. Of course, use of a small amount of the hydroxyl-containing trisulfide also may introduce a small number of disulfide linkages into the hydroxyl-containing liquid polymer, and these linkages are particularly subject to adverse effects of U.V. light.
It has been found that increasing the amount of hydroxyl-containing disulfide and trisulfide relative to other monomers results in hydroxyl-containing polymers which produce pressure-sensitive adhesives having improved 180.degree. peel strength but reduced U.V. stability. New pressure-sensitive adhesives are desired having improved 180.degree. peel strength without sacrifice of U.V. stability.